Liquid crystals are used primarily as dielectrics in indicating devices, as the optical properties of such substances can be influenced by an applied voltage. Electro-optical devices based on liquid crystals are well-known to the person skilled in the art and can be based on various effects such as, for example, dynamic scattering, the deformation of aligned phases (DAP cells), the Schadt-Helfrich effect (TN cells twisted-nematic and STN cells [super twisted-nematic]), the guest/host effect (guest/host cells). a cholesteric-nematic phase transition (phase-change cells) or the SBE effect (super birefringence effect). The response times of such indicating devices generally lie in the order of several milliseconds or more.
It has recently been found that the speed of response can be significantly improved by using indicating devices based on liquid crystals with ferroelectric properties. In this case there can be used basically various chiral smectic liquid crystals with ferroelectric properties such as, for example, liquid crystals with smectic C, F or I phases However, liquid crystals with chiral smectic C phases have been found to be particularly suitable.
Hitherto, comparatively few liquid crystals with ferroelectric properties have become known. Moreover, the known liquid crystals frequently have an inadequate chemical and thermal stability and have only relatively narrow chiral smectic phases or chiral smectic phases lying at elevated temperatures. There accordingly exists a great need for further suitable materials, especially having regard to the further improvement of ferroelectric liquid crystal mixtures.